1. Field of the invention
The invention concerns a method of transmission in a UMTS mobile telephone network, especially when operating in time-division duplex (TDD) mode. The UMTS system is the third generation cellular mobile telephone system. The GSM system will continue to exist for some time and the intention is to produce terminals adapted to use the two systems alternately. It must therefore be possible for such terminals to be handed over from a UMTS cell to a GSM cell during a call as the terminal moves around. One of the decision criteria for handover from one cell to another is the power of the signal received by the mobile terminal.
2. Description of the prior art
One known method of handing over a GSM terminal from one GSM cell to another during a call consists in:
having each base transceiver station send a continuous signal on a broadcast control channel (BCCH) carrying synchronization and frequency correction signals specific to one GSM cell, and
having each GSM mobile terminal receive the BCCH transmitted in GSM cells adjoining the GSM cell in which it is located, measure the power of any GSM signals received and prepare for possible handover by pre-synchronizing to an adjoining GSM cell.
Each GSM mobile terminal is capable of carrying out a power measurement whilst continuing a call set up in the cell in which it is located. It carries out the measurement in a free time slot provided between an uplink burst and a downlink burst. A GSM type system uses 120 ms multiframes each made up of 26 frames having the same duration, each frame being made up of eight time slots. In each frame an uplink burst occupies a certain number of time slots and a downlink burst occupies a certain number of the remaining time slots. The gap between an uplink burst and a downlink burst generally has a duration of two or four time slots, with one notable exception: in the 26th frame of each multiframe, the gap has a duration corresponding to 12 time slots, that is to say 6 ms. This 6 ms gap enables a portion of the pre-synchronization process to be performed every 120 ms. Pre-synchronization consists in decoding the frequency correction signals and the synchronization signals. The frequency correction and synchronization signals are distributed through a second type of multiframe corresponding to 51 frames. Consequently, pre-synchronization can take up to 1.3 s, with an average duration in the order of 0.6 s.
One transmission method currently under consideration for the UMTS system is the time-division multiplex (TDM) method. It uses frames having identical durations, each frame being in turn made up of time slots each having a given length. A first portion of each frame is allocated to an uplink burst and the remainder of the frame is allocated to a downlink burst, or vice-versa. The maximum number of time slots allocated to the uplink burst and the maximum number of time slots allocated to the downlink burst are fixed and identical for all the terminals, or at least for all the terminals situated in a cluster of UMTS cells. The ratio between the two numbers is chosen in accordance with the nature of the foreseen traffic. If the foreseen traffic is symmetrical, then this proportion is a priori equal to 50%. The proportion can be greater than 50% if the foreseen traffic is strongly asymmetrical.
At present there is no provision for preparing for handover from a UMTS cell to a GSM cell. It is feasible to produce a terminal including two receivers operating simultaneously, a UMTS receiver being used for a UMTS call while a GSM receiver prepares for handover to a GSM cell, but this solution would increase the cost and size of UMTS terminals.
The aim of the invention is to propose a solution that does not necessitate two receivers.
The invention consists in a method of transmission in a UMTS mobile telephone network enabling preparation for handover to a GSM cell during a call in a UMTS cell, the method consisting in transporting information in frames each made up of a plurality of time slots, time slots from each frame being allocated to mobile telephone terminals, the method further consisting in, for a given terminal, and at intervals equal to an integer multiple of the duration of a frame, not allocating to the terminal considered at least one continuous series of time slots whose total duration is at least equal to a predetermined time period enabling reception of the broadcast control channel from a GSM base transceiver station and pre-synchronization to that GSM base transceiver station.
The above method enables one receiver to handle a UMTS call and prepare for handover to a GSM cell quasi-simultaneously, using the gap available in at least some UMTS frames.
In one particular embodiment, the method in accordance with the invention further consists in reducing the spectral spreading of other time slots carrying data in order to increase their transport capacity.
The above method enables the use of a single receiver even if the terminal is used for a high bit rate service, for example to receive video signals or files, because the method increases the transport capacity of the slots allocated and therefore increases the number of free slots until a gap is obtained of sufficient duration to measure the power of any GSM signals received and prepare for possible handover by pre-synchronizing to an adjoining GSM cell.